The invention relates to a new process for measuring the zeta-potential on the surface of a material suspended in a liquid medium. The measuring is carried out, at simultaneously regulated flow velocity and voltage values, in a visually controlled measuring pipe provided with two electrodes.
According to the experiences of Smoluchowski, M. [Handbuch der Elektrizitat und Magnetismus, Ed. L. Gratz, (Barth Verlag, Leipzig 1921) Volume II, page 366] if a material suspended in liquid is subjected to the effect of steady voltage, the particles migrate. Based on the electrophoretic velocity (migration speed) (v.sub.e =cm.s.sup.-1) and the electric voltage density applied (E=V.cm.sup.-1, wherein V=voltage in Volts and cm=the distance between the electrodes in cm) the zeta-potential (ZP) characteristic for the material can be determined, while the positive or negative polarity thereof depends on the direction of the migration of the particles towards the cathode or the anode. Zeta-potential can be calculated on the basis of the following formulae: ##EQU1## wherein .eta. stands for the viscosity of the liquid medium, .epsilon. represents dielectric permittivity and 9.10.sup.4 is the conversion factor of the electrostatic unit into Volts.
In case of water, A=150 at a temperature of 20.degree. C. (Martin, A. N. Swarbrick, J. Cammarata, A: Physical Pharmacy) (Lea and Febiger, Philadelphia 1969, page 458). Determination of ZP on the basis of electrophoretic velocity is carried out in a measuring cell provided with molybdenum or platinum electrodes placed under a special colloidal microscope. The migration speed of the particles can be determined under stable D.C. voltage of known value after a certain time, in a stationary state, by means of a stop-watch, and an ocular micrometer, whereafter the ZP value can be calculated according to the aforementioned formula.
The unavoidable disadvantage of this widely used method is that in the course of electrophoresis, in particular in a medium which also comprises electrolytes, the temperature of the system continuously changes. Thermic flow considerably disturbs electrophoretic migration. In addition, gas evolves on the electrodes, electrochemical processes occur, and in the measuring cell, ions migrate, which causes a deleterious effect on the accuracy of the measurement. The fact that the liquid medium and the particles migrate in the opposite direction along the wall of the cell due to electroendoosmosis makes the measurement especially difficult. Along the vertical wall of the measuring cell the particles migrate with a changing velocity having a parabolic velocity profile. According to said velocity profile, particle velocity reaches its maximum at the axis of the measuring cell, while at the limit of endoosmosis it equals zero. Between this limit and the wall of the measuring cell the particles migrate in the opposite direction.
From the foregoing it is obvious that measuring of ZP on the basis of the electrophoretic migration speed is very difficult. Complicated apparatus is required; in addition, measuring errors frequently exceed 10%.
In the prior art several other ZP measuring methods are known. These include the method based on measuring the electroosmosis formed under the influence of electric field of force (Biefer, G. J., Mason S. G.: Colloid Sci. 9, 20) (1954); or on measuring flow potential (among others Martin, W. McK., Gortner, R. A.: J. Phys. Chem. 34 1509) (1930); or on the basis of measuring volumetric flow or potential of sedimentation. According to specific embodiments of these techniques, the measurement may be carried out in double microcapillary tubes in an electrophoretic cell; or using the phenomena of zone-electrophoresis, the relationship of moving phase borders and mass flow; or by electrophoretic light dispersion, laser technique (Robert J. Hunter: Zeta Potential in Colloid Science (Acad. Press London, 1981), pages 127-175). However, such apparatus is either not commercially available or if available, it is very expensive. Additionally, these methods of determining ZP are laborious and time consuming. Moreover, accuracy of measurement is limited to a variation coefficient of .+-.5%. In several professional fields, among others in the chemical and pharmaceutical industry, when plant-protecting agents, cosmetics and drugs are produced, or when the filtering processes are optimized in the chemical industry, sewage treatment, and drinking water purification, the accurate determination of the ZP-value of colloids and suspensions is critical.
Stabilization of suspensions and prevention of the aggregation of the suspended particles is possible by regulation of the ZP-value and proper choice of the additives on basis of ZP-measurements. In technical literature it has been generally accepted that suspensions with a ZP-value of -100 nV are extraordinarily stable. In order to increase efficiency of filtering processes (the aggregation of the particles) the ZP-value should be reduced. Proper choice of additives or conditions also requires measuring of the ZP-value.
An object of the invention is to improve the known techniques of measuring of ZP-value in order to develop a process which is easy to conduct, requires less work, and provides accurate measurements.